Continued investigation of the new technique of ion microscopy is proposed for direct elemental and isotopic histochemical analysis of biological tissues. Appropriate methodology will be developed to allow the use of ion microscopy for the study of important biomedical problems. Methodology studies include improvement of sample preparation methods to preserve the original elemental distributions, the investigation of the cesium ion source for increased sensitivity, and the feasibility of analyzing fluid samples and cell cultures in addition to thin sections. Digital image processing will be used for quantification of both features and fields of view in both two-and three-dimensional modes. The technique of isotopic tagging of both inorganic and biochemical species using enriched stable isotopes will be explored for ion microscopic localization at the cellular level. Appropriate methodology will be developed and applied to nutritional and other biomedical studies. A number of fundamental and applied collaborative studies are proposed representing a broad scope of biomedical areas. They include the role of calcium in chromosomes during mitosis, the regulatory role of calcium in polymorphonuclear neutrophils where these phagocytic cells are involved in defense against infection, the role of calcium in the molecular mechanisms of visual transduction in vertebrate retinal rod photoreceptors, and the role of calcium in epithelial function. Additional biomedical applications will be explored as time and experience permit.